Hybrid roof covering element

ABSTRACT

A hybrid roof covering element, which suitable for simultaneously heating a medium and generating electricity, and which comprises a single or multiple transparent layer, a flexible thin film solar cell sheet with a heat capacity of less than 3.5 kJ/m 2 K and a thermally insulating material, and a medium to be heated. The flexible thin film solar cell sheet comprises a carrier, a back electrode, a photovoltaic layer, and a transparent conductive front electrode and has a heat capacity of less than 3.5 kJ/m 2 K. This hybrid roof covering element has a response speed of more than 5.7·10 −4  K/J if the medium to be heated is air and a response speed of more than 1·10 −4  K/J if the medium to be heated is water.

The invention pertains to a hybrid roof covering element comprising athin film solar cell sheet. Hybrid roof covering elements are elementswhich can suitably be mounted on or in roof structures or otherwiseintegrated into buildings or, if so desired, in stand-alone systems, inwhich solar energy is employed in two ways. On the one hand, solarenergy is converted into electricity by a thin film solar cell sheet. Onthe other hand, solar energy is used to heat a gaseous and/or liquidmedium, such as air and/or water.

Hybrid roof covering elements comprising a thin film solar cell sheetare known. They are described, e.g., in U.S. Pat. No. 5,589,006 and EP 0820 105. U.S. Pat. No. 5,849,006 describes a hybrid roof coveringelement comprising, from top to bottom, a top layer composed of,preferably, a transparent coating, a thin film solar cell sheet cast ina filler, a back insulating member, a filler layer, and a roofing sheet.Air is passed underneath the roofing sheet. EP 0 820 105 describes asystem where a thin film solar cell sheet is provided on a back plate ofhigh load bearing capacity and high processability, so that the platecan be freely bent into any desired shape, more specifically into atrapezoidal shape. The air is passed underneath the back plate and overthe thin film solar cell sheet.

Although these hybrid roof covering elements function adequately, theyare open to improvement, especially as regards heat generationefficiency.

It has now been found that an improvement can easily be effected in thisfield by directly contacting a thin film solar cell sheet with low heatcapacity with the medium to be heated. This will result in higherefficiency on the part of the roof covering element, as less energy isrequired to heat the thin film solar cell sheet itself, leaving moreenergy for useful processing. This leads to a higher response speed onthe part of the hybrid roof covering element.

The invention thus pertains to a hybrid roof covering element comprisinga thin film solar cell sheet that is characterized in that it has a heatcapacity of less than 3.5 kJ/m²·K, preferably less than 600 J/m²·K.

The thin film solar cell sheet used in the hybrid roof covering elementaccording to the invention is a flexible thin film solar cell sheet. Theflexibility of the film is attractive for a number of reasons. First ofall, said flexibility makes it possible to transport the thin film solarcell sheet in rolled-up form to the place where the roof coveringelements are assembled. Secondly, it is easier to divide flexible thinfilm solar cells up into sections of the desired size than rigid thinfilm solar cells on, say, a glass carrier. Thirdly, the flexibility ofthe thin film solar cell sheet makes it possible for the film to beintegrated into the roof covering element in different ways if sodesired, as a result of which the position of the film vis-à-vis the suncan be optimized and any desired aesthetic effects may be obtained. Thusa flexible thin film solar cell sheet can not only be made into a flatsheet as is most common, but also bent into, say, a trapezoidal or someother desired shape. Thin film solar cell sheets manufactured by meansof a roll-to-roll process are particularly attractive, as they veryfully satisfy the desired properties of easy transportation and easydividing up into sections of the desired size.

The thin film solar cell sheet used in the hybrid roof covering elementaccording to the invention generally is composed of a flexible carrier,a back electrode, a photovoltaic layer, and a transparent frontelectrode. If so desired, the thin film solar cell sheet may be providedwith one or more protective layers or other top layers. The heatcapacity of such a thin film solar cell sheet is determined principallyby the nature of the carrier and any protective and top layers present.In selecting these materials care has to be taken to ensure that thedesired properties as regards heat capacity are obtained.

The thin film solar cell sheet employed in the hybrid roof coveringelement according to the invention preferably has an overall thicknessof less than 1000 μm, more preferably of less than 500 μm, mostpreferably of less than 300 μm. Generally speaking, the thinner the thinfilm solar cell sheet, the lower its heat capacity will be.

The thin film solar cell sheet employed in the hybrid roof coveringelement according to the invention preferably has a weight per surfacearea of less than 1400 g/m², more preferably of less than 700 g/m².Generally speaking, the lighter the thin film solar cell sheet, thelower its heat capacity will be. Furthermore, lighter sheets may resultin simpler and less costly transportation and processing.

Suitable materials for the carrier layer of the thin film solar cellsheet include thermoplastic or thermosetting polymer films, combinationsthereof, and, optionally, fibre-reinforced variations thereon. Suitablethermosetting materials include polyimides, unsaturated polyesters,vinyl esters, SI, etc. The thermoplastic materials may be amorphous aswell as semi-crystalline. Examples of suitable amorphous thermoplasticsare PEI, PSU, PC, PPO, PES, PMMA, SI, PVC, PVDC, FEP, and various otherfluorine-containing polymers. Examples of suitable semi-crystallinematerials are PET, PEN, PEEK, PEKK, PP, and PTFE. Examples of suitableliquid-crystalline materials are PPTA (Twaron, Aramica, Kevlar (alltrade marks)). Use may be made of melt-extruded films as well assolution-, emulsion- or suspension-cast films. Biaxially drawn films asa rule will have superior mechanical properties. Metal films on which aninsulating (dielectric) top layer has been provided, or compositions ofpolymer and fiber reinforcement like glass fiber reinforced epoxy-mayalso serve as a carrier layer if so desired.

Polymeric “co-extruded” films provided with a thermoplastic adhesivelayer with a softening point below that of the carrier itself arepreferred. Optionally, the coextruded film is provided with ananti-diffusion layer of, e.g., aluminium or SiOx.

At present, preference is given to biaxially drawn polyesters,preferably ones provided with an inorganic anti-diffusion coating.

The thickness of the carrier preferably is 75 μm to 1 mm. Preferredranges are 100 μm to 600 μm and 150 μm to 300 μm. As indicated above,the use of a thin film solar cell sheet manufactured by means of aroll-to-roll process is attractive. An attractive roll-to-roll processfor the manufacture of a flexible thin film solar cell sheet comprisesthe following steps:

a. providing a temporary substrate

b. applying the transparent conductive electrode onto said substrate

c. applying the photovoltaic layer

d. applying the back electrode

e. applying a permanent carrier onto the back electrode

f. removing the temporary substrate.

g. optionally applying a transparent protective layer

The reason why this process is so attractive for manufacturing thin filmsolar cell sheets for use in hybrid roof covering elements is that theuse of the temporary substrate makes it possible for the front electrodeof the transparent conductive oxide, the photovoltaic layer, and theback electrode to be applied under such conditions as will produce asolar cell of good quality. The permanent carrier which is applied ontothe thin film solar cell sheet at a later stage can be selected suchthat it will readily satisfy the conditions imposed by specific use in ahybrid roof covering element. However, there is no need for thepermanent substrate to be resistant to the conditions, e.g., the hightemperature conditions, prevailing during the application of the frontelectrode, the photovoltaic layer, and the back electrode, as a resultof which there is greater freedom of choice when it comes to selectingthe permanent carrier.

For that reason the invention also pertains to hybrid roof coveringelement comprising a thin film solar cell sheet manufactured by aprocess comprising the aforesaid steps a-g. Examples of suitableprocesses are those described in WO 98/13882 and WO 99/49483.

The nature of the back electrode, the PV layer, and the transparentfront electrode are not crucial to the present invention.

The transparent front electrode as a rule will be a transparentconductive oxide (TCO). Examples of suitable transparent conductiveoxides are indium tin oxide, zinc oxide, zinc oxide doped withaluminium, fluor, or boron, cadmium sulphide, cadmium oxide, tin oxide,and, most preferably, F-doped SnO₂. Said last-mentioned transparentelectrode material is preferred, because it can form a desiredcrystalline surface with a columnar crystal structure when applied at atemperature above 400° C., preferably in the range of 500 to 600° C. Inaddition, it is more resistant to chemicals than the much-used indiumtin oxide. Also, it is far less costly.

In the present description the term “PV layer” or “photovoltaic layer”encompasses the entire system of layers needed to absorb light andgenerate electricity. Suitable layer configurations are known, as aremethods to apply them. For general prior art in this field reference maybe made to Yukinoro Kuwano, “Photovoltaic Cells,” Ullmann'sEncyclopedia, Vol.A20 (1992), 161, and “Solar Technology,” Ullmann'sEncyclopedia, Vol.A24 (1993), 369.

Various thin-film semi-conductors can be utilized in the manufacture ofthe PV layer. Examples are amorphous silicon (a-Si:H), microcrystallinesilicon, polycrystalline amorphous silicon carbide (a-SiC), amorphoussilicon germanium (a-SiGe), and a-SiGe:H. In addition, the PV layer inthe thin film solar cell sheet according to the invention may comprise,e.g., CIS (copper indium diselenide, CulnSe₂) PV cells, cadmiumtelluride cells, Cu(In,Ga)Se cells, ZnSe/CIS cells, ZnO/CIS cells,Mo/CIS/CdS/ZnO cells, and dye-sensitised cells. Stacked cells may alsobe applied.

Preferably, the PV layer is an amorphous silicon layer when the TCOcomprises a fluorine-doped tin oxide. In that case the PV layer as arule will comprise a set, or a plurality of sets of p-doped, intrinsic,and n-doped amorphous silicon layers, with the p-doped layers beingarranged on the side receiving the incident light.

The back electrode in the thin film solar cell sheet according to theinvention preferably serves both as reflector and as electrode. The backelectrode may comprise any suitable material having light reflectingproperties, preferably aluminium, silver, or a combination of layers ofboth. In the case of silver, it is preferred to first apply an adhesionpromoter layer. TiO₂ and ZnO are examples of suitable materials for anadhesion promoter layer and have the advantage of also possessingreflecting properties when applied in a suitable thickness, e.g., ofabout 80 nm.

The hybrid roof covering element according to the invention has a higherresponse speed than conventional hybrid roof covering elements where thethin film solar cell sheet has been cast in a filler or is present on arigid weatherproof carrier with load bearing capacity, e.g., made ofcopper plate. Response speed in this context means the rate at which thetemperature of the matter to be heated changes when the radiationintensity changes. The response speed thus is defined as the change intemperature per unit of time per change in net flux and has the unitK/J.

In the case of the hybrid roof covering element according to theinvention, it is possible to distinguish the response speed of the thinfilm solar cell sheet itself on the one hand and the response speed ofthe medium to which the heat is dissipated on the other. This lattervalue is also referred to as the response speed of the hybrid roofcovering element. The response speed of the thin film solar cell sheetitself is a first indication of the response speed of the hybrid roofcovering element. The thin film solar cell sheets employed in the hybridroof covering element according to the invention preferably have aresponse speed of more than 5.7·10⁻⁴ K/J, more preferably of more than11·10⁻⁴ K/J. When the medium in the hybrid roof covering element to beheated is air, the response speed of the roof covering elementpreferably is more than 5.7·10⁻⁴ K/J, more preferably more than 11·10⁻⁴K/J. When the medium in the hybrid roof covering element to be heated iswater, the response speed of the roof covering element preferably ismore than 1·10⁻⁴ K/J, more preferably more than 2·10⁻⁴ K/J.

The hybrid roof covering element according to the invention can beconstructed in different forms. It may be suitable for the simultaneousgeneration of electricity and hot water; alternatively, it may besuitable for the simultaneous generation of electricity and hot air.Depending on the individual circumstances, other gaseous or liquid mediamay also be heated.

The hybrid roof covering element according to the invention maycomprise, for example, from top to bottom, a single or multipletransparent layer, a thin film solar cell sheet, and a thermallyinsulating material. Between the transparent layer and the thin filmsolar cell sheet and/or between the thin film solar cell sheet and thethermally insulating material there is a space comprising the medium tobe heated, such as air. In this case the single or multiple transparentlayer preferably has a transparency to visible light of more than 70%and a k-value of less than 4 W/m².K. The k-value is a parameterconventionally used in the art for the isolation performance of panels.

The hybrid roof covering element according to the invention is describedin greater detail below with reference to a roof covering elementsuitable for heating air, but it will be evident to the skilled personthat the heating of different media can be carried out in acorresponding manner.

In a hybrid roof covering element according to the invention which issuitable for the simultaneous generation of electricity and hot air, thethin film solar cell sheet is present in a space into which cold air isintroduced on one side while hot air is discharged on the other. Theair's supply and discharge may result from natural draught or be broughtabout by a ventilating plant.

In the roof covering element the thin film solar cell sheet may bemounted such that it is suspended freely, so that air will pass over thethin film solar cell sheet on either side. If so desired, the thin filmsolar cell sheet may be mounted on a rigid substrate with goodinsulating properties in the hybrid roof covering element. In saidlatter case, the heat capacity of the substrate should be so low thatthe combined heat capacity of the thin film solar cell sheet and therigid substrate does not exceed 3000 J/m²K. Preferably, the combinedheat capacity of the thin film solar cell sheet and the rigid substrateis less than 900 J/m²K, more preferably less than 450 J/m²K.

As regards energy efficiency, it is preferred to mount the thin filmsolar cell sheet in an air chamber, so that the thin film solar cellsheet will be in contact with the air flowing past on either side. Forreasons of firmness the use of a rigid substrate may be desired. Therigid substrate is made of, e.g., (foamed) plastic or a similar materialand constructed in the thinnest possible form.

The air chamber has to be insulated sufficiently also on the sidereceiving the incident sunlight, this in order to ensure that the heatgenerated is not dissipated to the outside air. This can be achieved byproviding a suitable insulating top layer. As described above, this toplayer preferably has a transparency to visible light of more than 70%and a k-value of less than 4 W/m².K. As was observed earlier, there isno need for the thin film solar cell sheet to be mounted flatly. It canalso be mounted shaped in a certain way, e.g., as disclosed in EP 0 820105, or in any other manner.

The most efficient way of obtaining hot water using a hybrid roofcovering element according to the invention is to generate hot air inthe hybrid roof covering element and use this to generate hot water withthe aid of a heat exchanger.

EXAMPLE 1

A hybrid roof covering element for the simultaneous generation ofelectricity and hot air is built up of the following elements, from topto bottom: a sealing against the effects of the weather, a transparentinsulating cavity plate of plastic underneath which air can flow, a thinfilm solar cell sheet on a foam plastic insulating layer, and a roofinglayer. The roof covering element is provided with inlets and outlets forcold and hot air, respectively. The air is heated in the space betweenthe thin film solar cell sheet and the transparent insulating plasticplate.

The thin film solar cell sheet comprises a plastic carrier withsuperimposed thereon an aluminium back electrode which also acts asreflector, a photovoltaic layer of amorphous silicon, and a layer offluorine-doped tin oxide as transparent front electrode. The film iscovered with a protective top layer of polymer. The thin film solar cellsheet has a heat capacity of 440 J/m²K, a thickness of 250 μm, and aweight per surface area of 300 g/m². The thin film solar cell sheet andfoam plastic plate combined have a heat capacity of 2500 J/m²K.

The thin film solar cell sheet has a response speed of 2.2·10⁻⁴ K/J. Theroof covering element has a response speed of 6.7·10⁻⁴ K/J.

What is claimed is:
 1. A hybrid roof covering element, suitable forsimultaneously heating a medium and generating electricity, whichcomprises a single or multiple transparent layer, a flexible thin filmsolar cell sheet with a heat capacity of less than 3.5 kJ/m²K and athermally insulating material, and a medium to be heated, wherein theflexible thin film solar cell sheet comprises a carrier, a backelectrode, a photovoltaic layer, and a transparent conductive frontelectrode, and wherein the hybrid roof covering element has a responsespeed of more than 5.7·10⁻⁴ K/J if the medium to be heated is air and aresponse speed of more than 1·10⁻⁴ K/J if the medium to be heated iswater.
 2. A hybrid roof covering element according to claim 1 whereinthe flexible thin film solar cell sheet has weight per surface area ofless than 1400 grams per m².
 3. A hybrid roof covering element accordingto claim 1 wherein the flexible thin film solar cell sheet has weightper surface area of less than 700 grams per m².
 4. A hybrid roofcovering element according to claim 1 wherein the flexible thin filmsolar cell sheet has a thickness of less than 1000 μm.
 5. A hybrid roofcovering element according to claim 1 wherein the flexible thin filmsolar cell sheet has a thickness of less than 500 μm.
 6. A hybrid roofcovering element according to claim 1 wherein the flexible thin filmsolar cell sheet has a thickness of less than 250 μm.
 7. A hybrid roofcovering element according to claim 4 wherein the carrier has athickness of 75 μm to less than 1 mm.
 8. A hybrid roof covering elementaccording to claim 4 wherein the carrier has a thickness of 100 μm to600 μm.
 9. A hybrid roof covering element according to claim 4 or 5wherein the carrier has a thickness of 150 μm to 300 μm.
 10. A hybridroof covering element according to claim 1 wherein the flexible thinfilm solar cell sheet is manufactured by means of a roll-to-rollprocess.
 11. A hybrid roof covering element according to claim 10wherein the flexible thin film solar cell sheet is manufactured by meansof a process comprising the following steps providing a temporarysubstrate, applying the transparent conductive front electrode onto saidsubstrate, applying the photovoltaic layer, applying the back electrode,applying the carrier onto the back electrode, removing the temporarysubstrate, and optionally applying a transparent protective layer.
 12. Ahybrid roof covering element according to claim 1 wherein between thetransparent layer and the thin film solar cell sheet and/or between thethin film solar cell sheet and the thermally insulating material thereis a space which comprises the medium to be heated.
 13. A hybrid roofcovering element according to claim 1 wherein the single or multipletransparent layer has a transparency to visible light of more than 70%and a k-value of less than 4 W/m²·K.
 14. A hybrid roof covering elementaccording to claim 1 wherein the medium to be heated is air and theenergy from the air heated in the hybrid roof covering element is usedto heat water with the aid of a heat exchanger.